Geeetech

Hello,

I've noticed an underextrusion problem in my printer. I've already replaced extruder and nozzle of my original Geeetech Prusa i3 Pro B, so it's unlikely to be related to that specific model. That's why my question is more generic I guess.

I noticed underextrusion in one single use case - printing a cylinder with 1.2mm thick wall (0.4mm nozzle). There's no such problem when printing straight walls (also 1.2mm thinkess).

Underextrusion happens after nozzle travel, when it starts printing a curve straight after that. There's absolutely no dependency on Z-hop and retraction (I tried printing with and without them with exactly same results). It only happens after travel - all the rest of the layer is printed fine with no artifacts, so unlikely to be related to any bubbles or anything else related to PLA condition. I tried different PLAs as well.

I use Ultimaker Cura 4.7.1 as a slicer and Repetier Server running on Raspberry Pi as a host program.

Given that it only happens on certain types of lines, it makes me think that there might be a bug in the slicer or some settings, but I'm not sure. May be there's something that I can check in the generated GCode?

Have you by any chance seen any similar behavior or have any ideas regarding what could be causing this?

Thanks!

Just posting an update to my old post, in case it becomes helpful to someone else.

Originally, I said that my underextrusion was happening on curved parts only, however later I observed it on other parts as well. After trying all sorts of alterations (firmware, filament, extruder, hot end, slicer, jerk settings, linear advance, cooling, retraction, speed, extra prime, flow) and not seeing any difference, I observed one distinctive feature which seems to be the key points, but which I don't know how to fix yet.

So the problem is: The underextrusion happens after long periods of small extrusions.

The test is:
1) make a cylinder and slice it with Cura, no infill, wall thickness 0.4mm (nozzle), no skin. It will print just a tube with one-line wall and it will simply do it printing ring by ring. This prints perfect (the left one on the photo)
2) make a module of a tube 0.4mm thickness (basically same shape), tick "print thing walls" in Cura and slice. This once prints really badly (the right print on the photo). All layers are screwed up expect the first one. The difference is - in the second case, Cura always tries to make the second inner line with a reeeeaally low extrusion flow. And even though I know that the first way is the correct one, but it really highlights the issue in general.

It looks like this micro steps in extrusion commands accumulate into a floating point operation error inside the firmware and as result, when it comes to the actual extrusion, the E parameter is already too high and thus it skips some extrusion commands in the beginning of the next layer print.

The solution which I found in the Cura was "Minimum Wall Flow", which by default is 0%. I put it 50% for starters which seems to prevented Cura trying to print some lines which don't make the difference in the printed model, but screw up E position calculation.

Hope it might be helpful to someone